Scripture records that there were about 120 people in the upper room at the time of Pentacost.
When they entered the city they went to the upper room where they were staying, Peter and John and James and Andrew, Philip and Thomas, Bartholomew and Matthew, James son of Alphaeus, Simon the Zealot, and Judas son of James. All these devoted themselves with one accord to prayer, together with some women, and Mary the mother of Jesus, and his brothers. During those days Peter stood up in the midst of the brothers (there was a group of about one hundred and twenty persons in the one place). (Acts 1:13-15)
One hundred twenty was the number of people required to start a new city. If you wanted to go off to start your own city with its own synagogue, you had to have 120 people willing to do so. You could not start a new Church unless you had 120 people.
Scripture is specific in mentioning that 120 people were present and the reference would have been clear to first century Jews who read the passage. It was clearly a reference to starting a new Church. All 120 present at Pentacost were Jews who believed that Christ was the Messiah.
I note with great joy that the author mentioned the names of the leaders of that Church and the fact that the Blessed Virgin was with them at the very start of the Church.
-Tim-